Such a method to determine a scheduled rate value, such a policing method and such a policing device are already known in the art, e.g. from the Contribution to the ITU Standard I.371 with reference number D.1104. This contribution is entitled `Proposed Text on Conformance Definition for the ABR Service` and is filed in the name of United States of America. In the annex from page 3 to page 6 of this document, a sample algorithm for policing ABR (Available Bit Rate) connections is described. In such ABR connections, the origin node, i.e. a source or virtual source node, regularly sends so called resource management (RM) cells in between the ATM (Asynchronous Transfer Mode) cells that contain data. These RM cells are interpreted by and their contents may be modified by network nodes, virtual destination nodes and/or destination nodes. Each resource management cell becomes reflected by a network or destination node to be returned to the origin node whereby it was sent. Thus, forward resource management cells (FRM) generated by an origin node become backward resource management cells (BRM) once they are reflected by a network or destination node. Based on the contents of the so received backward RM cells and rules defined in the section 5.10.4 on pages 51-52 of the ATM Forum Traffic Management Specification Version 4, published in February 1996, the origin node is supposed to control its transmit rate. According to the ABR specifications, a traffic contract specifies negotiated parameters such as the peak cell rate (PCR), the minimum cell rate (MCR), . . . , and probably the conformance definition which the network provider and the subscriber mutually have to support on the connections covered by this contract. A policing device in the network determines whether or not the traffic descriptors of the traffic contract are met and thereto applies the DGCRA (Dynamic Generic Cell Rate Algorithm) suggested by the ATM Forum that works on ABR standardisation. The policing device thereto schedules rate values and associated time values. From the scheduled rate and time values, the policing device can determine the maximum acceptable cell transmit rate of an origin node at each instant, called the actual policing rate in the remainder of this document. The above mentioned Contribution to the ITU Standard contains, from page 3 (last two lines) to page 4, line 8, a software implementation of an algorithm that determines a scheduled rate value r(i). To determine this scheduled rate value r(i), rate control information CI(i), NI(i) ER(i) of a backward resource management cell with index i is used. According to source behaviour rule 5, defined on page 52 of the above cited ATM Forum Traffic Management Specification, the cell transmit rate of a source or virtual source has to be forced to the initial cell rate ICR specified in the traffic contract whenever this source or virtual source sends a new resource management cell while a certain time period has elapsed since the last forward resource management cell was sent by this source. Moreover, rule 6 on page 52 specifies that the cell transmit rate of a source or virtual source has to be decreased in proportion to a predetermined rate decrease factor RDF if at least a certain number, CRM, forward resource management cells are received by the policing device since the last backward resource management cell was sent from the policing device to this source or virtual source. In the method disclosed in the above mentioned contribution, an origin node can return at a high transmission rate after a period wherein its cell transmit rate is forced to decrease in accordance to rule 5 or rule 6. This is so because the scheduled rate r(i) is calculated solely on the basis of the contents of backward resource management cells. It is apparent to persons skilled in the art of network traffic management that this may lead to congestion somewhere in the network.
In addition, the known method suffers some minor disadvantages. The policing method disclosed in the earlier mentioned Contribution to the ITU Standard for instance does not check whether the congestion indication (CI) bit, a bit which indicates that the source transmit rate in an ABR connection has to be decreased, is set in backward resource management cells. As a result the known policing method does not realise the tightest policing conditions for ABR connections. The known method also does not test cell sequence integrity. Rule 3 on page 51 of the already cited ATM Forum Traffic Management Specification, specifies the minimum number of ATM data cells that has to be sent between two successive forward RM cells, and the conditions that have to be fulfilled for transmission of a backward RM cell from the policing device towards the source or virtual source. If the policing device does not check cell sequence integrity, forward resource management cells may be flooding the network or may be sent too infrequently. It is however to be noticed (see point 3 in the paragraph 5.10.6 entitled `Switch behaviour` on page 54 of the ATM Forum Specification) that cell sequence integrity may be violated for data cells if the network contains a switching node between the origin node and policing device. Yet another shortcoming of the known algorithm is that it does not check out-of-rate cell conformance. Cells marked as out-of-rate cells are subjected to rule 11 on page 52 of the already cited Specification. When the policing device does not check conformance of such cells, these out-of-rate cells may be flooding the network and cause congestion.